Darren Ambrose’s first goal in a Town shirt in 12 years proved enough to take this third round FA Cup tie back to Portman Road for a replay.
The midfielder, whose short term contract at the Club is up next week in what is his third spell at Town, headed in from Tyrone Mings’ cross to give Blues the lead on 19 minutes.
Southampton levelled before the break through Morgan Schneiderlin and although they pressed for a winner late on, Town held firm to make sure of a place in the draw for the fourth round.
A Blue Army contingent of 5,500 made the trip to St Mary’s for the third round tie, with Mick McCarthy making six changes from the side that beat Charlton in midweek.
Dean Gerken started in goal, with former Southampton Academy player Tyrone Mings returning to the side. Stephen Hunt, Luke Hyam and Kevin Bru also came in, with Ambrose handed his first start in his third spell as a Blues’ player.
David McGoldrick, another ex Saint, had the first shot of intent but the danger was smothered on the edge of his box by Florin Gardos.
Bru fired wide from the angle, Hunt almost finding a way through as Town started the game brightly, Victor Wanyama going close at the other end with a shot from 30 yards.
Bru’s shot went straight at Fraser Forster after good work by McGoldrick in the best opening of the first 15 minutes, Mings then seeing his fierce cross hacked clear before the Blue Army were off their feet, Mings’ cross met by Ambrose whose header beat Forster and landed in the roof of the net.
Saints responded, Graziano Pelle backheeled just wide from Ryan Bertrand’s cross, then it needed the feet of Gerken to prevent Steven Davis equalising when clean through, Wanyama failing to get enough on the rebound to find the unguarded net.
Daryl Murphy escaped inside the Southampton box but his cross was cleared at full stretch by Jose Fonte, Bru almost releasing McGoldrick on goal seconds later.
Gerken did superbly to beat out James Ward-Prowse’s free-kick low down to his right but the ‘keeper was beaten on 33 minutes, Schneiderlin forcing the ball in from close range after getting between Christophe Berra and Gerken.
McGoldrick couldn’t find the supporting Ambrose as Town broke, Bertrand seeing his left foot volley deflected wide, with Hyam fizzing a shot just wide in the last effort of an even first 45 minutes.
Shane Long went down in the box under a challenge from Mings inside the first minute of the second-half but referee Martin Atkinson didn’t take a second glance, pointing for a goalkick as the home fans called for a penalty.
Smith did brilliantly to deny Pelle at the near post after Hunt’s misplaced pass, Murphy doing well to thwart the danger of Bertrand as he burst into the box.
Gerken gathered from Pelle’s strike from the edge of the box before Town made a first change just past the hour, Cameron Stewart coming on for his debut to replace Hunt.
Berra threw himself in the way of another Pelle effort 18 yards out as the home side turned up the heat on Blues’ goal.
Lloyd Isgrove came on for Dusan Tadic, Cole Skuse for Ambrose as the game entered the final 20 minutes with a place in the fourth round still very much up for grabs.
It was Luke Chambers this time who stopped Jose Fonte’s volley from close range, while at the other end Stewart sent in a cross that was cleared as McGoldrick waited to pounce.
Conor Sammon joined the action for the closing stages, Bru making way and although Saints pressed for a late winner, Town stayed resilient to the final whistle, with a replay to be played on Wednesday, January 14.